Laser assisted de-bonding is a pertinent process for temporary wafer handling in complementary metal oxide semiconductor (CMOS) processing and the like. Referring to FIG. 1, the de-bonding process involves a temporary glass wafer 102 that is bonded to the device wafer 104 using high-temperature adhesive polymer 106. The initial state is shown at 112. This approach facilitates further device processing on a thin device wafer. A through-glass laser ablation step is performed to separate (de-bond) the temporary glass wafer from the device wafer. The laser is seen at 118 and is omitted from views 114, 116. The de-bonding process relies on high-power pulsed lasers to ablate a thin polymer layer at the interface. State 114 shows a gap 108 in polymer 106 resulting from ablation. State 116 is the end state wherein the glass carrier 102 and wafer 104 are separated as seen at 110. Laser ablation is a non-linear process with a threshold requiring a minimum power for ablation. Excess laser power or an excess number of pulses may cause damage to underlying devices (laser ablation produces a shock wave that propagates through the polymer). A relatively small laser spot size must be scanned across the entire wafer. The current process requires precise control of laser fluence (energy per unit area) and avoiding exposure to excessive total pulses.
Thus, temporary wafer bonding and de-bonding are pertinent enabling processes for 2.5D and 3D packaging technology. A glass handling wafer is temporarily bonded to a workpiece (e.g. thinned device wafer) using an adhesive polymer to enable further processing. Laser de-bonding is a pertinent technology used to degrade the adhesive polymer layer and to separate the workpiece from the temporary glass wafer.
U.S. Pat. No. 5,258,236, hereby expressly incorporated herein by reference in its entirety for all purposes, teaches the laser assisted de-bonding process. A high power pulsed laser is directed to the polymer/glass interface through the glass wafer. The laser is used to ablate a thin polymer layer at the interface. The laser ablation process is a nonlinear process that relies on high-power pulsed lasers and requires a minimum power (threshold) to degrade the polymer. However, since high power lasers are required, excess laser power or an excess number of pulses may cause damage to the underlying devices in the workpiece.
B. Dang at al., “CMOS compatible thin wafer processing using temporary mechanical wafer, adhesive and laser release of thin chips/wafers for 3D integration,” Electronic Components and Technology Conference (ECTC), 2010 Proceedings 60th, pp. 1393-1398, 1-4 Jun. 2010, is hereby expressly incorporated herein by reference in its entirety for all purposes. Furthermore, P. Andry et al., “Advanced Wafer Bonding and Laser De-bonding,” Electronic Components and Technology Conference (ECTC), 2014 Proceedings 64th, pp. 883-887, May 2014, is hereby expressly incorporated herein by reference in its entirety for all purposes. The aforementioned B. Dang at al. and P. Andry et al. papers show modifications and further examples of the laser de-bonding process, although these may still be prone to potential damage problems as discussed herein.
Referring to FIG. 2, consider aspects of lasers such as laser 118 used for the ablation process. The laser beam will typically have a Gaussian profile 202 where the fluence will vary across the beam profile. Suppose the threshold fluence is 0.1 J/cm2; only the portion of the beam above the line 204 (0.1-0.2 J/cm2) has the required fluence; the region below (0-0.1 J/cm2) does not—see notation 208. As indicated by arrow 206, the beam is scanned across the region to be irradiated. Arrow 210 shows scanning to the right while arrow 212 shows scanning back to the left. Regions 214, 216, 218 show the irradiated regions from the pulses with at least the minimum value of the fluence on the rightward scan, while regions 224, 222, 220 show the irradiated regions from the pulses with at least the minimum value of the fluence on the leftward scan. It can be seen that there is overlap and potential excess incident energy at the following locations:                1. right-hand edge of 214 overlaps left-hand edge of 216;        2. right-hand edge of 216 overlaps left-hand edge of 218;        3. right-hand edge of 222 overlaps left-hand edge of 224;        4. right-hand edge of 220 overlaps left-hand edge of 222;        5. bottoms of 214, 216, 218 respectively overlap tops of 220, 222, 224.        
It will thus be appreciated that it is difficult to achieve a uniform dose (particularity with a non-linear threshold process).
Typical lasers used in the ablation process include high power pulsed ultraviolet (UV) lasers (a few nsec pulse duration in some cases; in some cases, say 10 nsec). This is a nonlinear process with an approximately 100 mJ/cm2 fluence threshold for the ablation of polymers. The dose is not cumulative; only pulses (or portions of pulses) above the threshold will ablate polymer. Multiple pulses will continue to ablate polymer leading to possible damage.
The wavelength should have good transmission in the glass wafer 102. Common lasers include excimer lasers (e.g., XeCl at 308 nm and XeF at 351 nm) and a Tripled Q-switched 1064 nm Nd:YAG laser (3rd harmonic at 355 nm).
Excimer lasers require complex optics to deal with their highly non-uniform beam profile. These include laser beam homogenizers and complex optical systems to deliver the beam to the target, as required for high-cost, high maintenance Excimer lasers. An approximately one cm spot size is scanned across the target.
Nd:YAG lasers require precise control of power and scan parameters to ensure minimum dose applied. They exhibit a Gaussian beam profile (Non-uniform) such that fluence varies across the beam profile. Precision control of the scanned Gaussian beam profile is required. Power control of the highly non-linear 3rd harmonic is required. Nd:YAG lasers are lower power than excimer lasers and typically have about a 0.2 mm spot size used in a high-speed raster scan across the target. Nd:YAG lasers are typically lower cost, low-maintenance lasers.
Referring now to FIG. 3, in a current process, the laser will continue to impinge on polymer 106 after polymer 106 delaminates from glass 102. Power is reduced by only about 4% due to reflection at the glass/“air” interface, and the laser will continue to ablate the polymer 106 for multiple pulses. Similar elements in the figures have received the same reference character. In particular, view 302 shows the first pulse wherein beam 308 impinges (typically near-normally) on polymer 106. Subsequent pulses of the beam (e.g. 310) are shown in view 304. In region 306, the polymer has ablated and delaminated from the glass creating an “air” gap; the above-mentioned glass/“air” interface is at the top of region 306. Please note that throughout this patent application, references to the “air” gap should be understood to include air, vapor or gaseous material resulting from ablation of the polymer, a mixture of air and vapor or gaseous material resulting from ablation of the polymer; a gas or gas mixture other than air (e.g., when process is carried out in an environment other than normal atmosphere), and/or a mixture of a gas or gas mixture other than air and vapor or gaseous material resulting from ablation of the polymer.